A common scheme for recreating a carrier for demodulating a received PSK signal is the provision of a phase-locked loop through which a carrier frequency is acquired and held, whereby synchronous detection of the received PSK signals can be carried out. In order to acquire the carrier frequency, it is quite often necessary to sweep the carrier recovery loop across a range of frequency uncertainty until lock is achieved; usually this frequency sweep proceeds automatically with a lock detector terminating the sweep upon sensing a lock condition. The U.S. Pat. to Walker et al No. 4,000,476, and Anderson et al No. 3,090,735, may be referred to for a description of exemplary signal recovery systems wherein such phase-locked loop circuitry is employed. In the above-identified Walker et al patent, the problem of side-locking is described, with the inventive system disclosed therein employing a lock inhibiter circuit which is used to drive the loop oscillator away from a side band frequency erroneously acquired by the loop. However, Walker appears to address the case of side locking to data related side bands and would not be totally effective against jammer-induced AM which is not data related; unless band pass filter 20 in Walker FIG. 3 happened to be tuned to exactly the pulse rate of the jammer, side lock would not be detected.
Unfortunately, side locking is one form of jamming to which a PSK demodulator is vulnerable, even when the transmitted PSK signal has been spread out by mixing the original information-containing signal with a pseudo random noise sequence prior to transmission, to thereby widen or spread out the spectrum of the transmitted signal energy. Because of the energy spread, the signal-to-noise ratio is substantially reduced, so that the input portion of the receiver correlation circuitry which collapses the spread spectrum signal into its original waveform bandwidth normally includes automatic gain control circuitry which establishes the necessary signal levels so as to maximize the received signal processing operation. If a strong interference signal, such as a pulsating jamming signal, is received along with the transmitted PSK signal, the influence of the jamming signal on the operation of the AGC circuitry may be such that it causes the AGC circuit to effectively amplitude-modulate the desired input signal. This amplitude modulation (AM), in turn, produces sidebands in the spectrum of the received signal, spaced at multiples of the jamming pulse rate. As a result, when the phase-locked loop, in its search for the carrier, sweeps across one of these sidebands, the lock detector, which cannot distinguish between a jammer-induced AM sideband and the true carrier, locks onto the sideband and terminates the frequency sweep, preventing the receiver from demodulating the information-containing signal.